1. Field of the Invention
This invention generally relates to a brake apparatus for a motorcycle and, more particularly, toward a combined brake system in which the front brake apparatus and rear brake apparatus are interlocked.
2. Description of Related Art
A brake apparatus of the by-wire type (BBW: brake by wire) is practically used. This brake apparatus (BBW) detects a brake operation amount and generates fluid pressure with a fluid pressure modulator based on the detected value so that braking force is generated from the fluid pressure. With the brake by wire apparatus (BBW) described above, an ABS (Antilock Brake System) function can be exhibited.
Further, a system is known wherein front and rear brake apparatus are interlocked with each other by operation of one of the front and rear brake operating elements in addition to the ABS function. In this regard, refer to, for example, Japanese Patent Laid-Open No. 2006-175993 (FIGS. 1 and 3).
A system wherein front and rear brake apparatus are interlocked with each other is called CBS (Combined Brake System), and the CBS function is selected by changing a mode changeover switch 32, shown in FIG. 1 of Japanese Patent Laid-Open No. 2006-175993, and then front wheel braking force and rear wheel braking force are controlled based on a fixed correlation, as illustrated in FIG. 3 of Japanese Patent Laid-Open No. 2006-175993.
In this manner, in the conventional CBS function, the front-rear distribution of braking force is determined uniquely. However, from a point of view of a rider, since a front-rear distribution upon forward traveling, for example, in such a case that the μ of the road surface is so high that high braking force of a tire can be expected, a distribution in the case where importance is attached to vehicle body control upon turning, and a front-rear distribution in the case where the road surface μ is low are different from one another in response to a variation of the traveling environment, such as traveling on a public road and traveling circuit, and a variation of road surface condition, such as a dry road surface and a wet road surface, there is room for further improvement in order to be ready for a variation of the front-rear distribution of braking force in response to various environments because there is a demand to change the front-rear distribution in response to changes in the traveling environment.
In the case where braking force is generated also by the rear wheel in an interlocking relationship with a braking operation for the front wheel, although basically the vehicle should be decelerated at a deceleration conforming to an ideal vehicle body deceleration, since, in the vehicle body, the load principally on the front wheel side is inclined to become higher than that on the rear wheel side by braking and, on the rear wheel side, the rear wheel side load decreases by sink of the suspension for the front wheel side, in the case where front-rear interlocking braking is to be carried out, the braking force to the rear wheel side is sometimes changed over from increase to decrease rather early. In this instance, if the braking force to the rear wheel side changes over from increase to decrease although the braking force to the front wheel side increases, then this results in variation of the increasing degree (inclination or rate of change) of the deceleration as a vehicle body, and the linearity of the deceleration tends to fade. Thus, there is a case in which adjustment of the vehicle body deceleration by a braking operation for the front wheel becomes difficult.